Preparation of aryl dicarboxylic acids



J. L. DARRAGH ETAL PREPARATION OF ARYI.. DICARBOXYLIC ACIDS Aug. 14,1951 2 Sheets-Sheet l Filed.. Oct. 4 1947 A TTORNEYS Aug. 14, 1951 J.l.. DARRAGH ETAL 2,563,820

PREPARATION OF ARYL DICARBOXYLIC ACIDS Filed oct. 4, 1947 z sheets-sheet2 h air, s a H n r m w H m m WM. N D J mmntumzw zocbm w. L. ,n oom oohoom o?I m n 6 3 .n w m w H L ...u $32515 w oom oom. oov oom W o nm mm 1d 7. 3 H .A n mw a .l N G l. 9 E E. a A N Z M o .3 l I D 9 mm V H l w Gam TTORNEYS Patented Aug. 14, 1951 PREPARATION 0F ARYL DICARBOXYLICACIDS John L. umani and mmm J. Miner, Berkeley,

Calif., assignors to California Research Corporation, San Francisco,Calif., a corporation of Delaware Application October 4, 1947, SerialNo. 777,970

9 Claims. l

This invention relates to the preparation o! aryl carboxylic acids inwhich a carboxyl group preferably is directly attached to the aromaticnucleus and involves the production of such aryl carboxylic acids fromalkyl aromatic hydrocarbons and preferably from polymethyl-substitutedaromatic hydrocarbons. More particularly the invention relates to aprocess for producing mono nuclear polycarboxylic acids, and in itspreferred embodiment comprises a process for preparing isomeric phthalicacids, such as the terephthalic isomer.

Known methods for producing isomeric phthalic acids, such asterephthalic, have utilized indirect chemical syntheses which result incosts so high as to be prohibitive for large scale production. Certainmore direct methods such as oxidation of para xylene with potassiumpermanganate or chromic acid require consumption of relatively expensivechemical reagents `and likewise involve excessive costs.

An object of this invention is to provide a new and improved process forthe production of aryl carboxylic acids and one which is of specialadvantage for preparing isomeric phthalic acids. Another object is tofurnish an improved method for the synthesis of terephthalic acid.

Additionally, an object is to synthesize mono nuclear arylpolycarboxylic acids with low cost chemical reagents.

Another object is to produce terephthalic acid by a process whichinvolves oxidation with caustic Ialkali. Further, an object is toprovide a process capable of enhancing yields of carboxylic acids in thecaustic oxidation of aryl methyl chlorides.

Other objects and advantages of the invention will be apparent from thedrawing and the following detailed disclosure.

In accordance with this invention aryl carboxylic acids, particularlymono nuclear aryl polycarboxylic acids, are obtained by formation ofpolyalkyl 4aromatic chlorides in which a chlorine atom is attached to aprimary alkyl carbon atom, and conversion of such chlorides tocarboxylic acids by reactions involving oxidation with aqueous causticalkali. Preferably, the chlorinated primary alkyl carbon atom isattached directly to a benzene nucleus, as in chlorides of para xylene:

omc] calci cHcl. ouch Hi glzcl gli: H101 -X -l l PX l Iene p-Xylalp-Chlcromethyl ghloxie dichyloide chloride benul chloride However, theprocess is applicable to compounds such as CHzCHaCl CHiCHzCl CHxCHsCl;H3 Hz Cl Hr-CHCI CHiCl CHzOl CHzCl and their homologs. Correspondingderivatives of naphthalene and anthracene also may be utilized.

Briefly described a process embodying the present invention utilizes acombination of process steps involving: I

(l) Formation of an aryl substituted primary alkyl chloride such asxylyl chloride xylyl chloride (CHaCeHiCHzClz), xylylene dichloride(CICHzCaHiCHzCl) or mixtures thereof, either by chlorination of xyleneor by chloromethylation of toluene or both;

(2) Separation of at least a xylylene dichloride fraction from thechlorinated reaction mixture;

(3) Conversion of the separated xylene chlorides to correspondingcarboxylic acids by hydrolysis and oxidation with aqueous causticalkali.

(i)4 chlorination of at least a part of the resulting toluic acids tocorresponding chloromethy1 benzoic acids, and

(5) Conversion of chloromethyl benzoic acids to the correspondingdicarboxylic acids by hydrolysis and oxidation with caustic alkali.

It should be noted that the chemicals consumed in the foregoingsynthesis are chlorine and aqueous caustic alkali, each of which is alow cost bulk chemical. In chloromethylation formaldehyde also isconsumed, and this likewise is a low cost bulk product. No expensiveoxidizing agents are required. Also, it is important that chlorinationto a single chlorinated product is not necessary to substantiallycomplete conversion to phthalic acids.

Since the process of this invention finds its principal presentapplication in the manufacture of terephthalic and/or isophthalic acids.the invention and process will be illustrated hereinafter by referenceto'the production of these isomeric phthalic acids from the xylenesand/or toluene. Special emphasis is herein given to the production ofterephthalic acid from paraxylene.

In the drawing, Fig. 1 is a flow sheet illustrating in block diagram aprocess for converting para xylene to terephthalic acid or for obtaininga mixture vof phthalic acids from toluene or for producing mixedphthalic with or without benzoic acid from xylene and toluene.

Fig. 2 is a graphical showing variation with temperature (in F.) ofyield of terephthalic acid and terephthalic content of acids fromcaustic oxidation of chloromethyl benzoic acid.

Fig. 3 is a similar graph showing effect of temperature (in F.) on yieldof benzoic acid from benzyl chloride.

Reference to the drawing will reveal that a xylene fraction such as paraxylene is introduced by way of feed line I to a chlorination zone Ilwhere chlorine is introduced into the methyl side chains. Thechlorination reaction is conducted under conditions which favor sidechain chlorination and may result in the following reactions:

CH; CHzCl heat i Ch HC1+ (p-xylyl chloride) light Hx Ha CHgCl CHsCl heatCl; HC1+ (p-xylylene dichloride) light Hs BaC1 CHzCl CHOI:

heat C11 HC1+ (p-xylal chloride) light Ha Ha Triand polychlorinatedcompounds also may be formed. If minor amounts of ring chlorinationoccurs, the subsequent process steps still yield the desired compounds.The mixture of chlorinated products then passes as indicated by line I2to a separation zone i3 and, as here shown, three fractions ofchlorinated xylenes are obtained. Any suitable method of separation.such as distillation under relatively high vacuum (to minimizedecomposition of the chlorides), may be used. When the hydrocarbon feedis para xylene, a para-xylyl chloride fraction, a para-xylal chloridefraction and a para-xylylene dichloride fraction are conducted as shownby lines i4, I6 and II respectively to further processing units. Ifbenzyl chloride is formed from any toluene present in the xylene feed,this chloride is removed by way of line 29. Lower boiling fractions,such as unchlorinated hydrocarbons, may be removed through outlet 30 andhigher boiling compounds are withdrawn as indiacted byline 35.

The para xylyl chloride fraction from line' Il is converted to paratoluic acid by reactions involving hydrolysis and oxidation of thehydrolyzed product to toluic acid.

lo CHxCl CH|0H Nuon Naci Hg HI onion cooN Nuon 2N,

Hg Hl These reactions may be eilected either substan- H CHCh =0 COON:

CHO

l Hl Ha After being converted from salt to the free 55 acid the paratoluic acid from zones Il and 2i next is subjected to side chainchlorination to form chloromethyl benzoic acids by the reactions:

CHI Q Cl: 00H

(Molten) CHgG] Q HC1 00H Side reactions also may occur, for example:

CHClz Q 21101 oon CH: C Ch son, anon oon H Minor chlorination in thering is not precluded.

Although the para toluic acid may be chlorinated while dissolved insolvents such as carbon tetrachloride, better results have been obtainedby direct chlorination of the molten acid, for example. at temperaturesof lfrom 190 C. to 270 C.

The foregoing conversion to chloromethyl benzoic acid is effected inreaction zone 22 from which the chlorinated products are conveyed asshown by line 23 to reaction zone 2l for conversion to terephthalicacid. In this reaction zone the chloromethyl groups are hydrolyzed andthen oxidized by aqueous caustic alkali to a carboxyl group. therebyyielding terephthalic acid by the following reactions:

H501 mon C|}OONa CIJOONa H2OH OONa Corresponding reactions occur withdichloro methyl toluic and trichloro methyl toluic acids.

The reaction mixture containing terephthalic acid passes to productrecovery zone 26. Any suitable method of recovery and purification maybe used, for example, acidication, crystallization and washing withsolvents.

When and if a mixture of xylenes is fed to chlorination zone Ii, acorresponding mixture of ortho, meta and para xylene chloride isomerswill be obtained. The product will therefore be mixed isomeric phthalicacids; but, if desired, a separation between para toluic acid on the onehand and meta and ortho toluic acids on the other hand may be elected,as by distillation. Two different phthalic acid products, namely aterephthalic acid and a mixture of ortho and meta phthalic acid, maythen be produced from the two separated toluic acid fractions. Aftersuch a separation (not shown in flow sheet), the mixed ortho and metatoluic acid fraction may be withdrawn as indicated by line 21 fromcaustic oxidiation zone I9. The resulting para toluic passes to zone 22lfor chlorination as shown. The xylylene dichlorides may be separated insuitable fashion. Thus, because of its high melting point the paraxylylene dichloride may be separated by crystallization. The ortho andmeta isomers may be separated from each other by fractionaldistillation, preferably under vacuum to avoid decomposition.

Toluene which may be contained in the mixed xylene feed or separatelyintroduced by way of valve-controlled line 28 will be chlorinated inzone Il to form benzyl chloride which is conducted to separation zone I3and withdrawn 6 therefrom as a benzyl chloride fraction indicated byline 29. The benzyl chloride fraction is converted to a mixture ofxylylene dichlorides by chloromethylatlon in zone 3|. The xylylenedichloride product is hydrolyzed to the corres' reaction zone 36. Thehydrogen chloride formedin xylene chlorination zone Il together withformaldehyde is utilized as shown to effect the chloromethylationreaction. A mixture of ortho, meta and para xylyl chlorides fromreaction zone 36 is passed by way of line 31 to separation zone 38. Ifdesired the xylyl chlorides may be fractionated and a puried para xylylchloride fraction removed from separation zone 38 by way of line 40.'When a mixed xylene hydrocarbon feed is used for zone Il, only mixedchlorides need be separated in zone 36. Hydrolysis to methyl benzylalcohol in reaction zone I3 and conversion to a mixture of toluic acidsin reaction zone IS is effected as previously disclosed. If and when itis desirable to synthesize a relatively pure terephthallc acid, themixed toluic acids are fractionated to separate a para toluic productand leave a mixed ortho and meta toluic acid fraction as previouslydisclosed. The para toluic acid fraction will then be passed tochlorination zone 22 for conversion to para chloromethyl benzoic acid.The chloromethyl benzoic acid then passes as shown by line '.'3 tocaustic oxidation zone 24 where it is converted to terephthalic acid inthe manner hereinbefore disclosed.

The process of this invention and suitable modes of operation will bereadily apparent in the light of the following illustrative data andexamples.

In small scale operations, the chlorination reaction of zone Il wascarried out in glass reactor equipped with a sintered glass plate at thebottom through which gaseous chlorine: was fed. The reactor when usedfor continuous chlorination was provided with an inlet line at thebottom, through which the hydrocarbon feed entered the reactor and anoutlet line near the top for removal of reaction mixture at the samerate as hydrocarbon entered the reaction zone. Reaction temperatureswere taken by a thermocouple extending into the reaction liquid. Thereactor was topped by a water-cooled condenser through which exit gaseswere removed and absorbed in aqueous caustic. Vacuum dstillationsutilized for separating the xylene chlorides were carried out indistillation columns packed with glass helices and equipped with anelectrically heated jacket. Liquid constituents of the reaction mixtureswere removed through an ordinary water-cooled head. Para xylylenedichloride and other high melting constituents were taken off thefractionating column through a special head surrounded by a glass jacketcontaining boiling toluene to prevent solidication in the lines prior toreaching the storage receiver.

Chiorination in batch operation can be effected in the presence of lightat approximately C. This temperature may be varied from 4') to C., forexample. When theoretical quanaseaseo titles of chlorine necessary toform a xylylene dichlorlde were passed into the reaction mixture, somedehydrohalogenation occurred both during chlorination and upondistillation of the chlorinated product under vacuum at 3-8 mm. mercurypressure. Continuous chlorination was carried out at about 95 C. in thepresence of light with greatly improved results. Para xylene andchlorine were fed to the reactor at constant rates and excellentchlorine utilization obtained. Steady state compositions were obtainedfor each feed ratio in continuous chlorination by taking samples fromthe continuously withdrawn chlorinated product and analyzing forchlorine content. When chlorine content became constant.

the reactor was assumed to be under steady state TABLE i Distillationanalysis of chlorinated p-zylene Distillation No 1 3 4 Results (Per Centby We Loss l.

From the results of the foregoing distillations, it will be seen that asthe chlorine to para xylene feed ratios are increased the yield of .paraxylylene di'chloride increases.

A large batch of the reaction mixture of chlorinated compounds wastreated for separation as follows: The chlorinated mixture was cooled inan ice bath and the crystals formed were filtered off. The nitrate wasthen charged to a still and unreacted para xylene and para xylylchloride removed under vacuum. The still bottoms were dissolvedvin abouthalf their volume of chloroform and the resulting solution cooled inanice bath where additional crystals were recovered and combined withthose rst separated. These crystals, after purification byrecrystallization from' chloroform, were identied as para xylylenedichloride by chlorine content, melting point and hydrolysis to the paraxylylene glycol derivative.

The mother liquor from the foregoing chloroform crystallization wasconcentrated to 50% of its volume and again cooled in ar ice bath. Asecond fraction of crystals more soluble in chloroform than paraxylylene dichloride was obtained and recrystallized from a mixture ofpetroleum ether and benzene. These crystals were identied as para xylalchloride by their chlorine content and melting point and by the factthat hydrolysis yielded para talualdehyde rather than the glycol.

After separation of the chlorinated reaction mixture to yield a xylylchloride fraction, a xylal chloride fraction and a para, xylylenedichloride fraction, these respective intermediates are treated forconversion to the corresponding carboxylic acids or sodium saltsthereof.

ooNvERsroN 0F xYLYL CBLORIDE A preferred mode of operation involvessimultaneous hydrolysis and oxidation of xylyl chloride to thecorresponding toluic acid at 300 C. to 400 C. (S75-750 FJ. The overallreaction may be written:

Beat zNaoH o,

COONa Naci 211,0

. CH: However, it may be effected in two stages:

CHlCl CHloH N 0H Heat N ci mo a CE: CH:

cHioH cooNa NaOH( Heat B o B H5 e .e o, q o, 2

CH3 CH;

A single stage operationpresently'is regarded as most desirable. Thereaction may be effected by agitation in a rocker type autoclave underthe following conditions:

Time, hours 1 Temperature, C 371 Caustic concentration, percent 8 NaOH,moles 2 Xylyl chloride, moles 1 Yield of toluic acid (moles per mole ofxylyl chloride) .85

XYLAL CHLORIDE CONVERSION Xylal chloride is converted to thecorresponding toluic acid by hydrolysis and oxidation preferably at 300C. to 400 C. (S75-'750 F.). These reactions may be effected by agitationin rocker type autoclave as follows:

CONVERSION 0F XYIJYLENE DICHLOR'IDE In small scale runs for this stageof the process reactions were carried out in a rocking autoclave capableof withstanding pressures up to 16,000 pounds per square inch gauge andconstructed entirely of Monel metal. The head of the vessel was ttedwith a thermocouple, a safety valve. vent, a pressure gauge and asamplevalve. Heat was supplied through strip heaters located in the autoclavecasing. The whole assembly was agitated by rocking in a vertical planewith'a maximum deviation from the horizontal of 30 and at a rate of 78cycles per minute.

,The desired material was charged to the autoclave together with a,predetermined amount of caustic soda, water and the oxidizing agent whenone was used. The vessel was lock-closed, placed in a shaker casing. andif a gaseous oxidizing agent auch as air or oxygen was to be used. itwel charged at this time through the sample valve. CHiCl CHioH Theamount of oxidizing agent was calculated from the total pressure andknown free-space in zNBOH 2mm the vessel. A

Heat was turned on and shaking started. After 5 HIC] H203 reachingreaction temperature, the mixture was allowed to4 react for one hourwhile controlling D'Xylylelmde p'll 31ml the temperature to maintain itsubstantially constant. At the end of one hour reaction time (ex elusiveof heating and cooling periods), shaking lo 2N0H 4H: was stopped, heatturned oil and the apparatus allowed to cool overnight. Upon removal ofthe HioH N., cooled reaction mixture from the autoclave, itTerephthalicacid was ltered to remove dirt and traces of insoluble Inany event it has been found that terephby-products and next acidied to apH of 3.0 with l5 thalic acid can be prepared from para xylylene HCl toconvert the salts back to free acid whichl forms a precipitate. Theprecipitated acid was recovered by filtration, washed free of chlorideion, dried and weighed. A sample of the dried product was dissolved in astandard base solution, back-titrated potentiometrically with standardacid and the acid number obtained. Yield was calculated from these data.

Pure para xylylene dchloride when reacted with four or more mols ofcaustic soda per mol of the dichloride at 371 C. (700 F.) for one houryields a terephthalic acid product containing impurities. Yields andimpurity contents are illustrated by the following table of data:

TABLE II Caustic oxidation of :cylylene ichloride glycol by subjectingthe glycol to caustic oxidation at 371 C. (700 F.) for one hour as inthe conversion of para xylylene dichloride. The product from the causticoxidation of para xylylene glycol contained a neutral product.paratoluic acid, and terephthalic acid substantially as in the previouspreparation from para xylylene dichloride. The xylylene glycol wasobtained for this reaction by hydrolysis in a dilute water solution atreflux temperature followed by extraction with ether to recover theglycol from the aqueous medium.

Important variables were investigated to determine their effects uponthe foregoing caustic Wt. Per Wt. Yield Mols Zy- Conc. Conv. Acid No.lylene Di- Nblsn NaOH Based on Bggltmt ggstn] of Product chloride UsedSolution, Chloride, Based on Charge mg. KOH Charged Per Cent Per CentCharge Per Cent per Gram 0. 50 2.0 8. l 100 l. 0 65 452 0. 50 2.0 8. 11(1) 53 410 D. 50 2. 0 8. l 100 2. 3 66 423 0. 50 2.0 8. 7 100 1. 7 64427 The by-product on which approximate peroxidation reactions. Amongthe factors invescentages are given above is a neutral, pasty materialinsoluble in aqueous caustic. The acid .numbers shown in the foregoingtables were obtained on the crude chloride-free acidic reactionproducts. Acid numbers can be raised to from about 500-512 in all casesby recrystallization from water. The significance of acid numbers isillustrated by theoretical acid numbers as follows:

Toluic acid -412 Terephthalic acid -672 Pure terephthalic and puretoluic acids were isolated from the reaction mixtures and identified bymeans of acid number and methyl ester; In addition a water insolubleneutral material was found to be present and is removed byk filtrationof aqueous solutions during recrystallization. The foregoing reaction isbelieved to occur step-wise as follows:

tigated were: effect of` concentration of the xylene chloride in thecaustic, effect of concentration of caustic and of excess proportions ofthe caustic solution on the product, effect oi' time and temperature ofreaction, and effect of oxidising reagents. Each of these factors isdiscussed hereinbelow. EFFECT or CONCENTRATION oN CONVERSION 0F XYLYLENEDiCHLoRIDE A decrease in concentration of the xylylene dichlorideincreases the acid number of the product and reduces the amount ofneutral by-product. The weight yield of the product remainssubstantially constant and the concentration as well as the amount ofexcess caustic over theoretical appeared to have no effect on theproduct within the limits investigated. The following series of data -inwhich the reactions were coni ducted at 700 F. with one hour reactiontime are illustrative:

TABLE III dichloride to terephthalic acid Wt. Per Wt. Yield Mols Xy-Mols Conc. Per Cent Cent of of Acids Acid No. lyiene'Di- NaOH NaQH Conv.Bypmdm,t Based on of Product chloride Used Solution, Based on Based onCham@ mg. KOH Charged Per Cent Chloride Charge Per Cnt per Gram 0. 102.0 8. l 100 truce 91 520 0. 02 2. 0 8. l 100 trace 69 577 0. 02 2. 0 8.l 100 trace 67 570 0.02 0.08 0.4 trace 63 572 a,ees,sao

Eirect of temperature, time, and pressure on turbo-mixer equipped with amercury-sealed .r1/Igiene dichloride conversion-A temperature stirrer.Chlorine was introduced at the bottom between 315-371 C. (60o-700 F.)yields a prodof the reactor which was immersed in an elecuct of a fairlyconstant acid number. As temtrically heated oil bath. The chlorine waspreperatures increased above 371 C. (700 F.) or 5 heated in a coiledfeed line immersed in the oil reaction times are prolonged, or both.theyield of bath and leading to the bottom of the reactor. total acid dropson. Thus, it appears that about Exit gases from the reaction mixture'pass 315 C.` (600 F.) is as high a temperature as is through a side-armto a condenser. The toluic necessary and that reaction times in excessof acids tend to sublime and plug air condensers.

one hour achieve little or no benefit. However. To alleviate thistendency an oil heated conthe process is operative at temperatures offrom denser was attached to the reactor and an air- 250 C. to 400 C.M80-750 F.). Illustrative data cooled condenser in turn attached to theoutlet are givenin the following table: of the oil heated condenser.Exit gases. thus.

TABLE IV Oxidation ol xylylene dichloridevariable time and temperatureWLPBI Wt. Yield M01" Mols. Conc' Cent of By- Acids Acid No X l iene NaOHTime Temp., Product Dicyh orlde Ngs? Solution Bours F. gragfdugfllageaggg? mg. KOH Charged Per Cent Charge Per Cent per Gram 002 1.0 4.21.0 550 14 a7 5m 0. 02 1.0 4.2 1. 0 600 None 77 584 0. 02 l. 0 4. 2 1. 0650 None 75 570 0. 02 l. 0 4. 2 4. 0 650 None 75 579 0. 02 2. 8 8. l 4.75 700 None 43 616 0. 02 l. 0 4. 2 1. 0 750 N one 23 573 0. 02 1. 0 4. 24. 25 750 None None Pressure is not critical except that it should befirst passed through the oil condenser where a suillcient to maintainthe aqueous caustic in major proportion ofA any sublimed acids wereliquid phase. condensed. melted and returned to the reactor.

Eect of oxidizing agents on conversion of The remaining gases nextflowed to the air con- .rylylene dichiarate- It has been found that usedenser and were led to a caustic absorber for of an oxidizing agent,such as air, achieves a extracting HC1 and residual acidic productscarmarked improvement in yields and purity of the ried thereby. yterephthalic acid product. In fact, substantially 4 Although para toluicacid, for example. may theoretical yields of nearly pure terephthalic bechlorinated in boiling carbon tetrachloride in acid have been obtainedby intimately contacting the presence of light, direct chlorination ofthe reaction mixture of aqueous caustic and molten toluic acids yieldsbetter results. The xylene chlorides with air. The data in the tabletoluic acid is charged to the reactor, melted, below exemplify theexcellent results obtainable. heated to the desired temperature andthere- TABLE V Caustic oxidation of .'rglylene dichloride in thepresence of added oxidizing agents Mols Mols Conc. Mols. of Per Cent ofMol Acid No. Tereph- Xylylene NaOH NaOH oxidizing Available TheoreticalYield Acids thalic Dichloride Used Solution, Agent Used Oxygen Oxygen ofmg. KOH in Prod., Charged Per Cent Charged Charged Acids `per Gram PerCent 0.02 0. 015 3.0 NaoGl 0.01 ss 023 s1 0.02 0.00 3.3 Naocl 0.02 10004 673 100 0.02 1.0 4.2 Air 0.08 400 as 674 100 0.25 1. a7 1.0 o, 0.40200 00 ses 9a All of the above runs were made at 315 C. after contactedwith gaseous chlorine under con- (600 F.) and one hour reaction time atthis temstant agitation. Temperatures of from 190 C. perature. to 270 C.are suitable conditions. While chlo- CONVERSION OF TOLUIC ACIDS rinationof para tolulc acid to form para methyl benzoic acid is a preferredaspect of the inven- Returning now to the intermediate toluic acids,tion since a main objective is to produce terephit will be recalledthat, in addition to use as thalic acid, it was found that all threeisomers such. these compounds may be converted to their of toluic acidwill undergo direct side-chain chlocorresponding phthalic acids bychlorination in rinationrin the molten state. Ortho toluic acid themethyl group and caustic alkali oxidation gave a 13.7%chlorine-containing product, for of the resulting chloromethyl benzoicacid example; and two runs with meta toluic acid (a chloro para toluicacid). Each of these stages yielded products containing 16.8 and 19.1%chlowill be described. s rine respectively. Various runs made with parachlorination of toluic acida-The direct chlotoluic acid and data onresults obtained are given rination of toluic acids was carried out in aglass in Table VI:

TABLE VI chlorination of p-toluic acid Per Cent Gm. of Per Cent AcidAcid No. Gm. Per Cent Tereph- Cla used Per Cent of Reacted No. slgleDescripcion or Charge "f 221' thune T'p" per glp. 0101. o1, whichPrfgl-n Chlgfme mg. KOH/m Acid m Tulum Beaded Dehydro- 0151.01@ weiyhtKon Charge Acid halogenated g Der gm i p-Toluic Acid (C. P.) 412 1. 19375 2 p-Toluic Acid Laboratory Prep. 414 .8 246-252 1.07 14.2 450 M. P.178-180.5 C. 3 p-Toluic Acid, Laboratory Prep. 414 0.8 246-252 1.07 13.0447 M. P. 178l80.5 C. 4 p-Toluic Acid, Laboratory Prep. 422 3.8 246-252.668 68 34 1.11 13 2 446 M. P. 177 220 C. 5 p-Toluic Acid, LaboratoryPrep. 414 .8 252 .884 73 44 1. l0 16.4 480 M. P. 178-180.5 C. 8 p-ToluicAcid, Laboratory Prep... 426 5. 3 246-252 .922 66 .56 96 14. 4 461 7p-Tolulc Acid, from Pilot Plant, 445 12.5 246-252 .838 79 55 1.08 13.7520 Caustic purified. 3 d0 Y 447 13. 3 232-241 982 82 52 1.09 17. 7 5629 p-Tolulo Acid, from Pilot Plant, 447 13.3 2552-241 .932 73 55 1.1114.1 560 l Caustic puriiled, air and chlorine used. 10 p-Toluic Acid,from Pilot Plant, 439 13.0 232-241 .882 80 66 .98 12.4 616 Steamstripped, containing l about 2% polymer.

,I u' Attention isV directed to the fact that in Table Theparachloromethyl toluic acid desirably is converted to terephthalic acidin a single stage caustic oxidation process by contacting with aqueouscaustic at 15G-400 C., for example at 315 C. (600 F.). The followingexamples are illustrative and show that chlorine content of the feedwithin the ranges tested has no profound effect upon the acid number ofthe product. A large excess of oxygen diminished the weight yield ofproduct.

TABLE VII Oxidation. of chlorinated p-toluic acid in caustic soda athigh-temperature Product chlorinated Chlorine Moles Moles NaOH Yield ofTolmc Acid m Charge, Oxygen Rm NO- charged, Per cem Nbsgg ruggrlt'charred la? Per Cent frffegf Acid No. Grams of Theory (as air) Per Cr'np-Toluic tehalric mgs. KOH by wt of Acid Acid per Gram Charge tions.While some of the para toluic acid charge stocks contained terephthalicacid, the amount 'therein is insufficient to account for the high acidnumber of the products. Extraction of the chlorinated products withproof alcohol (in which terephthalic acid is insoluble) yielded analcohol soluble portion and an alcohol insoluble portion having acidnumbers of 366 and 522, respectively. Accordingly, it is believed thatthe chlorine serves as an ,oxidizing agent to convert a portion of thepara toluic acid to terephthalic acid by an oxidation reaction which ispresently not understood.

On comparing sample 10 with samples 8 anri'9, it appears that removal ofimpurities from the crude toluic acid by caustic purification ordistillation achieves substantial improvements in chlorine utilization,and in yields and purity of product.

Conversion of para chloromethyl benzoc acid.-

The effect of different variables on the single stage conversion of parachloromethyl benzoic acid was investigated.

Eect of temperature on conversion of chloromethyl benzoic acida- Effectof temperature is decided in altering yields of total acids as well asper cent terephthalic acid in the reaction product. The effect ofchanges in temperature on total weight of acids and on the percentage ofTABLE VIII Elect of temperature on caustic oxidation ofa-chioro-p-toluic acid Product Omgimllimd M 1 Neon M0 0 C 05 0' Charged,Used Per Cent (as air) Per Cent Tonne Tere hmgs.

Grams by Wt o' d thal c KOH per charia .4cm Gram 5.0 1.o 4.2 0.04 300 tea1 sa 51s 5.o= 1.0 4.2 0.04 400 ss 22 1s 017 5.0 1.0 4.2 0.04 500 04 ns0 644 5.0 1.0 4.2 0.04 000 es a 07 ses Eect of concentrations onconversion of chloromethyl benzoic acid.-The foregoing runs wereperformed with the chlorinated toluic acids present in dilute solutionand with a high molar excess of caustic soda. Additional tests revealedthat the reaction can be eiected with good yields and relatively highetilciency' at high concentrations of the chlorinated acids and withoutlarge excess of caustic soda. given in Table 1X:

Illustrative data are 25 product.

their` corresponding phthalic acids by caustic oxidation at temperatureswithin the range of 150- 400 C., e. g. at 315 C. (600 F.), for areaction period of one hour in a manner similar to the foregoingexamples with para chloromethyl toluic acid. 'I'he invention andcondition for reaction as herein disclosed is applicable to the orthoand meta isomers as well as to the para Efect of causticconcentration-Relatively di- TABLE 1X Caustic oxidation ofn-chloro-p-toluic acid Product ghloiio- Chlicrine C o u c Mols one' MolsYield of Sample Acid Charge NaOH NaOH Oxy en Total Per Cent No. Charged,Per Cent Solution, g Per Cent Acid No.

Grams of Used Per Cent Charged ggf pTolulo Ttflxigl ms. KOH Theory wt ofAcid Acid per Gram Charge 57. 9 65 l. 5 5.6 0. 35 95 14 86 638 69. 5 66l. 5 5. 6 0. 37 93 8 92 654 103 85 2. 0 9. 9 0. 60 95 4 96 662 80.0 68l. 5 8.1 0.50 98 3 97 666 86. 6 59 l. 6 8.4 0.53 87 19 81 l 625 l Thisexperiment was made on a chlorinated crude toluic acid.

In the foregoing tests, oxygen was substituted for air so that lowerpressures could be utilized.

Eect of oxidizing agents- Except where otherwise noted, oxygen in theform of air was used for all of the reported examples on the conversionof para chloromethyl benzoic acid. It has been found that quantities ofoxygen less than one mol per mol of para chloromethyl benzoic. acid,although operative, result in loss in conversion of terephthalic acidand yield reaction products having lower acid numbers. A slight excessof oxygen is preferred as will be apparent from the following data:

TABLE X CONVERSION BY CHLOROMETHYLATION Utilization of by-product HC1 bychloromethylation of toluene or of benzyl chloride (produced bychlorination of toluene contained in or added to the xylene feed) hasbeen shown in the ded Use of oxygen in caustic oxidation ofa-chlorop-toluic acid to terephthalic acid Product Chloro- Chlorine MolsToluic Mols NaOH Yield of Samp] Acid i Charge NaOH Solution, OxygenTotal Per Cent No' Char ed Per Cent Used Per Cent Charged Acids Per PerC'-lt Tere l1 Amd No Gmls 0l' Theory (as air) Ceut'by pT0lu1c thac mgs.KOB WL .Acid Acid per Gram Charge Ortho chloromethyl toluic acid andmeta. scription of the iiow sheet of Fig. 1. This type of chloromethyltoluic acid have been converted to reaction will be illustrated inconnection with the 17 chloromethylation of benzyl chloride to form paraxylylene dichloride. The reaction CHN, CHIC] Reaction mix allowed tostand overnight, washed,

H dried, and ltered. Reaction produc-ts sepa- Hzo HC1 H20 5 alilsid byfractional distillation at reduced pres- Cmcl The/resulting product maybe converted to toluic acids as hereinbefore disclosed. is carried outin the presence of a condensation From the foregoing it will becomeapparent catalyst such as zinc chloride, hydrogen fluoride, that thecombination of process steps herein disor aluminum chloride. Thetendency of benzyl closed affords a method for obtaining substanchlorideto react with itself may be reduced by tially complete conversion of axylene, toluene or feeding the benzyl chloride to an excess of theanalogous methyl-substituted aromatic hydrocarreaction mixture and bysuch other expedients bon to corresponding carboxylic acids with high aswill occur to those skilled in the art. yields while avoiding thenecessity for complete The process is illustrated by small scaleexamchlcrination to theoretical dichloride content. It ples in whichbenzyl chloride, formaldehyde, catis unnecessary to obtain maximum yieldof the alyst, and solvent were mixed and contacted with i xylylenedichloride, for example, in order to pro- HCl for a period of severalhours. The reaction duce dicarboxylic acids thereby permittingundermixture was allowed to stand overnight, washed, chlorination withresulting higher selectivity for dried and ltered. Solvent and unreactedbenzyl side-chain chlorination as against ring chlorinachloride wereremoved by distillation and the dition while simultaneously minimizingoverchlo chloride was recovered by crystallization. Data rination in theside chain with attendant higher and results of these runs are given inTable XI: overall chlorine and caustic consumption. The

TABLE XI Data from chlormnethylation experiments Sample No 22 23 24 25Reactants:

Benzyl Chloride 1 m01 2 111018 1 mol l mol. Formaldehyde.- .54 eq. offormalin.-. 2.1 eq. of (CHi0)x.. l eq. of (CH2O)z. 1.33 eq. of (CHO x.Source oi' HC1 anhydrous HC1 anhydrous HC1. anhydrous HC1-.." 125 ml.conc. HCl. Catalyst,... 69 gm.l 138 gmJ 68 gm. Zl'iClz 60 ml. 85% H31Solvent 350 mi. Ethylene 500 mi. Ethylene 300 ml. Ethylene 110 gm.Glacial Ace Chloride. Chloride. Chloride. tic Acid. Temperature, C 50 5050 100. Time, Hours 8 9.. 8 4%, Analysis of Product: 2 n UrreagtedBenzyl Chloride, Per 1l.. 92, xy/Iieiie Diehloridarercent 30.. se. 3s.2. Polymers and Di-chloromethylated 27 25 42 Nil.

Products, Per Cent.

1 Catalyst prepared by fusing 210 gm. of ZnClz and stirring in 10 gm. ofA101; as it cooled.

2 Yields are based on the amount of formaldehyde used.

The xylylene dichloride may be converted to corresponding phthalic acidby either a single stage caustic oxidation or by first hydrolyzing toform the xylylene glycol and then oxidizing to the dicarboxylic acid.Although a single stage conversion usually is preferred, conditions fora twostage process will he illustrated. In the rst stage xylylenedichloride is subjected to hydrolysis in dilute water solution atrefiuxing temperature. The xylylene glycol is recovered by etherextraction of the reaction solution. In the second stage the recoveredglycol is subjected to caustic oxidation by intimately contacting theglycol with air or oxygen and an aqueous caustic solution at, forexample, 371 C. (700 F.) for one hour as disclosed in the variousprocesses for converting the chlorides to the acids. The resultingproduct will be a mixture of para toluic acid and terephthalic acidwhich can be separated in any suitable manner as by distillation. Ifdesired, the para toluic acid may be passed to a chlorination stage forconversion to para chloromethyl benzoic acid after which a causticoxidation will serve to form terephthalic acid therefrom.

Xylyl chloride is obtained from toluene by chloromethylation in a manneranalogous to the chloromethylation of benzyl chloride. Exemplaryconditions are:

Ethylene dichloride, ml 500 Toluene, moles 1 Zinc chloride, gr 68invention also furnishes a process for completely converting a mixtureof chlorinated methyl benzenes, such as xylenes, to carboxylic acidsnotwithstanding that the degree of chlorination or the number ofchlorine atoms per molecule of xylene varies widely. Despite thisvariation in chlorine content, production of corresponding carboxylicacid salts may be effected in single stage conversions. The process alsois adapted to produce polycarboxylic acids from the underchlorinatedcompounds by further chlorination and conversion of any monocarboxylicacids from the foregoing single stage conversions. The process enableseconomical utilization of by-product HCl produced in the chlorinationreactions. It is important that terephthalic and the like acids areobtained by this invention with relatively cheap bulk chemicals, such ascaustic soda, chlorine and, when desired, formaldehyde.

The benefits of various features of this invention are applicable tolthe caustic oxidation of benzyl chloride, benzyl alcohol orbenzaldehyde to benzoic acid. This is especially true concerning the useof air or gaseous oxygen to increase the yield of acid produced. Forexample, in comparing runs made at 371 C. (700 F'.), 3800 Pounds persquare inch, and a reaction time of one hour, it was found that byutilizing two mols of caustic in 8% aqueous solution per mol of benzylchloride without addition of air Yor oxygen, about 88% yield of benzoicacid was obtained with about 8% by-product. In a second run at 19 371 C.(700 F.) at the same pressure and reaction time, the results were asfollows:

Benzyl chloride reacted, mols 0.107

NaOH used, mols 0.225 Solution strength, percent 2 Mols of O2 (as air)0.12 Mol yield of acids, percent 96 Mol yield of by-product, percent 4Intimate contact oi the gaseous oxygen with the liquid reactants wasobtained by vigorous agitation. It will be observed that the presence ofthis oxidizing agent reduced the amount of byproduct formed from about 8to 4%, a factor of approximately one-half.

The eiiect of temperature on caustic oxidation of benzyl chloride tobenzoic acid is illustrated by the following discussion and data.

Fig. 3 reveals that reduction of temperature below 340 C. (650 F.)decreases yield of acid but does not render the process inoperative.Optimum reaction temperature for conversion of benzyl chloride tobenzoic acid is in the range of from S70-400 C. (700-750o F.).

When either benzyl alcohol or benzaldehyde was substituted for thebenzyl chloride under exactly the same reaction conditions as were usedfor the benzyl chloride conversion, somewhat higher yields oi the acidwere obtained.

Suitable temperatures for conversion of benzaldehyde and benzyl alcoholto benzoic acid by this reaction are 300 to 400 C.

Air or gaseous oxygen is advantageously used as an oxidizing agent inthe foregoing reactions. Caustic oxidation of benzyl chloride, benzylalcohol and benzaldehyde all produce the same by-product, mainly, benzylether, and in the case of benzyl chloride some unreacted benzyl alcohol.These by-products may be recycled to vmain reaction zone in order tosuppress further formal tion thereof and increase the yield of acid.

Reference has been made to the fact that when and if ring chlorinationoccurs as a side reaction, the desired carboxylic acid is neverthelessobtained. This factor is believed highly significant to the good yieldsand superior purity of the products herein obtained. Under the causticoxidation reaction conditions herein disclosed, the chlorine in the ringis removed and converted to HC1 initially, which isof course neutralizedby the caustic yielding a desired carboxylic acid. The removal of suchring chlorine is illustrated in conversion of chloro benzyl chloride tobenzoic acid in which the following reaction occurs:

CHxCl CO OH Analogous reactions occur with xylyl, xylal and xylylenechlorides containing chlorine in the ring as well as in the side chain.

20 Although the caustic alkali herein utilized for purposes ofillustration has been sodium hydroxide, other strong alkali hydroxidessuch as Apotassium hydroxide may be substituted therefor. Likewise othermodifications and variations of the invention as hereinbefore set forthmay be made without departing from the spirit and scope thereof. andonly such limitations should be imposed as are indicated in the appendedclaims.

We claim:

1. A process for producing benzene carboxylic acids which comprisesoxidizing a xylene having at least l and not more than 2 chlorine atomssubstituted in each methyl group by contact with aqueous caustic alkaliin an oxidation zone at a temperature in the range about 300 to 400 C.under a pressure suiiicient to maintain the aqueous caustic alkali inliquid phase to form a reaction product comprising benzene carboxylicacid salts and hydrogen.

2. The method as defined in claim 1, wherein a free oxygen-containinggas is introduced into the oxidation zone at reaction temperature toconsume produced hydrogen.

3. The method as defined in claim 2, wherein the chlorinated xyleneconsists predominantly of xylene dichloride.

4. The method as defined in claim 1, wherein the chlorinated xyleneconsists predominantly of xylene dichloride.

5. A process of producing a benzene dicarboxylic acid which comprisesmixing an aqueous caustic alkali with a chloromethyl benzoic acid`reducing pressure buildup from hydrogen so formed by intimatelycontacting said liquid phase mixture with a gas containing free oxygenwhile at said reaction temperature, and recovering benzene dicarboxylicacid from the reaction mixture ina form selected from the groupconsisting of the free acid and a salt thereof.

6. A process of producing terephthalic acid which comprises convertingpara-mono-chloromethyl benzoic acid to terephthalic acid by oxidationwith aqueous caustic alkali at a temperature of from to 400 C. and undersufficient pressure to maintain said aqueous caustic in liquid phase.

7. The method as defined in claim 6, wherein a free-oxygen-containinggas is introduced into the mixture of aqueous caustic alkali andparamono-chloromethyl benzoic acid at conversion temperature.

8. A process of producing terephthalic acid which comprises mixing anaqueous caustic alkali with para-xylylene dichloride, oxidizing saidxylylene dichloride to terephthalic acid with said aqueous causticalkali at a pressure sufficient to maintain at least a part of the waterin liquid phase and at a reaction temperature of from 300 to 400 C.whereby hydrogen formation occurs, reducing pressure buildup fromhydrogen so formed by intimately contacting said liquid phase mixturewith a gas containing free oxygen while at said reaction temperature andrecovering terephthalic acid from the reaction mixture in a assasno formselected from the group consisting of the free acid and a salt thereof.

9. A process of producing terephthalic acid from apara-di-(chloromethyl) benzene having no more than 2 hydrogen atoms ofthe methyl 5 groups substituted by chlorine, which comprises convertingsaid di-(chloromethyl) benzene to said acid by oxidation with aqueouscaustic alkali in liquid phase at 300 to 400 C. whereby hydrogenformation occurs, and reducing pressure buildup from hydrogen so formedby intimately contacting said liquid phase mixture with free oxygenduring said oxidation with caustic alkali and at a temperature of from300 to 400 C.

JOHN L. DARRAGH. ROBERT J. MILLER.

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1. A PROCESS FOR PRODUCING BENZENE CARBOXYLIC ACIDS WHICH COMPRISESOXIDIZING A XYLENE HAVING AT LEAST 1 AND NOT MORE THAN 2 CHLORINE ATOMSSUBSTITUTED IN EACH METHYL GROUP BY CONTACT WITH AQUEOUS CAUSTIC ALKALIIN AN OXIDATION ZONE AT A TEMPERATURE IN THE RANGE ABOUT 300 TO 400* C.UNDER A PRESSURE SUFFICIENT TO MAINTAIN THE AQUEOUS CAUSTIC ALKALI INLIQUID PHASE TO FORM A REACTION PRODUCT COMPRISING BENZENE CARBOXYLICACID SALTS AND HYDROGEN.